The invention pertains to a surface acoustic wave (SAW) device which forms part of an antitheft automotive system, as described in the aforementioned patent application Ser. No. 227,282. The previous application describes a SAW device comprising a coded SAW delay line and attached coil antenna that is embedded in or integrally formed with an otherwise conventional mechanical ignition key.
A coil mounted around the lock cylinder mounted in the steering column provides a brief radio frequency pulse. This pulse is picked up by the coil antenna, which excites a surface acoustical wave on the SAW delay line. After a time delay, the antenna is re-excited by a coded set of pulses determined by the arrangement of interdigital transducers (IDT) on the SAW delay line. These transponded pulses are picked up by the original coil, and processed electronically to ensure that the code on the SAW delay line matches a predetermined code. If there is no correct match, this is interpreted as an unauthorized intrusion and further action is inhibited.
A major problem that is encountered with SAW devices of this type is the existence of spurious signals. In the present context, the term spurious signal means unintended transponded signals that interfere with the correct code interpretation. These spurious signals arise from two sources: (1) crosstalk; and (2) double transit interference.
Crosstalk refers to output pulses created by launching and receiving SAWs within the subIDT structure, e.g., a SAW launched by a subIDT and received by another subIDT. Double transit interference refers to output pulses created by two SAW transits. For example, a SAW launched by the main IDT and received by a subIDT contributes to an output pulse; this output pulse excites the coil antenna as desired. However, acoustic energy is also reflected from a subIDT, and redirected and picked up by other subIDTs as well as the main IDT.
An initially developed coded SAW delay line based system for electronic labeling provided a means to reduce crosstalk spurious pulses by arranging the SAW delay line so that the SAW transit time from the main IDT to the first subIDT exceeded the total transit time from the first subIDT to the last subIDT..sup.1 In this way, all of the crosstalk spurious pulses arrived before the main, wanted, code-bearing pulses. It was recognized that it was important to provide additional means for suppressing spurious signals..sup.2 Means were devised using parallel acoustic wave paths to achieve this suppression.
Other means included the use of phase coding. The utilization of microwave frequency and decoding of a phase coded signal, however, requires complex and expensive electronics.
In an automotive antitheft system it is most important to reduce costs to a minimum. To realize this objective, the rf frequency should be below 50 MHz, so that low cost electronic components, currently available for other consumer devices, can be employed.
However, when the rf frequency is reduced below 50 MHz, spurious signals cannot be eliminated effectively by utilizing either parallel acoustic wave paths or phase coding techniques.
The restrictions on frequency, chip size and coding method (amplitude encoding) imposed by the automotive antitheft system with spurious signal elimination at the lower frequency leads to an unreasonably low number of available codes.
The present invention relaxes the previous requirement that the SAW device transit time from the main IDT to the first tap subIDT exceed the transit time from first subIDT to the last subIDT, thus increasing the number of available code bits. This provides a greater number of codes for an effective automotive antitheft system, but requires a new technique for eliminating or reducing spurious signals.
The current invention uses the following new techniques to eliminate spurious signals:
(1) time narrowing; PA1 (2) time shifting; PA1 (3) time spreading; and PA1 (4) cancellation.
Time narrowing uses narrower subIDTs with respect to the main IDT, thus diminishing crosstalk signals.
Time shifting utilizes the addition of a half code period to the spacing between the main IDT and the subIDTs, thus shifting the crosstalk signals between recognized signal centers.
Time spreading operates to shape the pattern of electrode overlaps within the subIDTs in order to spread crosstalk signals over a longer period of time, thus weakening these spurious signals.
Cancellation of the crosstalk signals is accomplished by adding additional IDTs, which generate additional signals of equal amplitude and time, and opposite phase, relative to the crosstalk signals.
Also contemplated by the present invention is the reduction or elimination of reflected acoustic energy resulting in double transit interference. This is accomplished by connecting an external resistor across the SAW device coil or by terminating the detection coil with a resistor.
Also additional IDTs can be connected to the bus bars which do not interact acoustically with the code-generating IDTs by the utilization of an acoustic absorber.